When exposed to high temperatures (i.e., greater than or equal to about 1,300° C.) and to oxidative environments, metals can oxidize, corrode, and become brittle. These environments are produced in turbines used for power generation applications. Thermal barrier coatings (TBC), when applied to metal turbine components, can reduce the effects that high-temperature, and corrosive and oxidative environments have on the metal components.
Thermal barrier coatings can comprise a metallic bond coating and a ceramic coating. The metal bond coating can comprise of oxidation resistant protective materials such as aluminum, chromium, aluminum alloys, and chromium alloys. For example, the metallic bond coating can comprise of chromium, aluminum, yttrium, or combinations of the forgoing, such as MCrAlY where M is nickel, cobalt, or iron (U.S. Pat. No. 4,034,142 to Hecht, and U.S. Pat. No. 4,585,481 to Gupta et al. describe some coating materials). These metallic bond coatings can be applied by thermal spraying techniques.
The family of thermal spray processes includes detonation gun deposition, high velocity oxy-fuel deposition (HVOF) and its variants such as high velocity air-fuel, plasma spray, flame spray, and electric wire arc spray. In most thermal coating processes a material in powder, wire, or rod form (e.g., metal) is heated to near or somewhat above its melting point and droplets of the material accelerated in a gas stream. The droplets are directed against the surface of a substrate to be coated where they adhere and flow into thin lamellar particles called splats.
In a typical detonation gun deposition process, a mixture of oxygen and a fuel such as acetylene along with a pulse of powder of the coating material is injected into a barrel, such as a barrel of about 25 millimeters (mm) in diameter and over a meter long. The gas mixture is detonated, and the detonation wave moving down the barrel heats the powder to near or somewhat above its melting point and accelerates it to a velocity of about 750 meters per second (m/sec). The molten, or nearly molten, droplets of material strike the surface of the substrate to be coated and flow into strongly bonded splats. After each detonation, the barrel is generally purged with an inert gas such as nitrogen, and the process repeated many times a second. Detonation gun coatings typically have a porosity of less than two volume percent with very high cohesive strength as well as very high bond strength to the substrate.
In high velocity oxy-fuel and related coating processes, oxygen, air or another source of oxygen, is used to bum a fuel such as hydrogen, propane, propylene, acetylene, or kerosene, in a combustion chamber and the gaseous combustion products allowed to expand through a nozzle. The gas velocity may be supersonic. Powdered coating material is injected into the nozzle and heated to near or above its melting point and accelerated to a relatively high velocity, such as up to about 600 m/sec. for some coating systems. The temperature and velocity of the gas stream through the nozzle, and ultimately the powder particles, can be controlled by varying the composition and flow rate of the gases or liquids into the gun. The molten particles impinge on the surface to be coated and flow into fairly densely packed splats that are well bonded to the substrate and each other.
In the plasma spray coating process a gas is partially ionized by an electric arc as it flows around a tungsten cathode and through a relatively short converging and diverging nozzle. The temperature of the plasma at its core may exceed 30,000 K and the velocity of the gas may be supersonic. Coating material, usually in the form of powder, is injected into the gas plasma and is heated to near or above its melting point and accelerated to a velocity that may reach about 600 m/sec. The rate of heat transfer to the coating material and the ultimate temperature of the coating material are a function of the flow rate and composition of the gas plasma as well as the torch design and powder injection technique. The molten particles are projected against the surface to be coated forming adherent splats.
In the flame spray coating process, oxygen and a fuel such as acetylene are combusted in a torch. Powder, wire, or rod, is injected into the flame where it is melted and accelerated. Particle velocities may reach about 300 m/sec. The maximum temperature of the gas and ultimately the coating material is a function of the flow rate and composition of the gases used and the torch design. Again, the molten particles are projected against the surface to be coated forming adherent splats.
Thermal spray coating processes have been used for many years to deposit layered coatings. These coatings consist of discrete layers of different composition and properties. For example, the coating may be a simple duplex coating consisting of a layer of a metal alloy such as nickel-chromium adjacent to the substrate with a layer of zirconia over it.
The coating processes can be used to apply thermal barrier coatings (TBC) and/or environmental barrier coatings (EBC) to components of turbines, engines, and the like, to protect the components from the harsh operating environments. To protect turbine components in these combustion environments, a class of coatings has been developed based on the formula MCrAlY where M represents a transition metal such as iron, cobalt, or nickel. A current problem exists when MCrAlY coatings are used in integrated gasification combined cycle (IGCC) systems. IGCC systems use an innovative process, which uses coal to produce power. The process is cleaner and more economically efficient than other processes that use coal to produce power. The process involves treating coal and reforming coal to a gas mixture that includes hydrogen gas (H2), carbon monoxide (CO), and carbon particulates. This gas mixture is combusted with oxygen in a turbine to produce power. The carbon particulates, however, collide with the coated turbine components and erode the components and/or coatings, and thereby shorten the effective operating life of the components.
Therefore, there exists a need for coatings that can provide improved protection for turbine components.